Emergence of Order in Origin-of-Life Scenarios on Mineral Surfaces: Polyglycine Chains on Silica

The polymerization of amino acids (AAs) to peptides on oxide surfaces has attracted interest owing to its high importance in biotechnology, prebiotic chemistry, and origin of life theories. However, its mechanism is still poorly understood. We tried to elucidate the reactivity of glycine (Gly) from the vapor phase on the surface of amorphous silica under controlled atmosphere at 160 °C. Infrared (IR) spectroscopy reveals that Gly functionalizes the silica surface through the formation of ester species, which represent, together with the weakly interacting silanols, crucial elements for monomers activation and polymerization. Once activated, β-turns start to form as initiators for the growth of long linear polypeptides (poly-Gly) chains, which elongate into ordered structures containing both β-sheet and helical conformations. The work also points to the role of water vapor in the formation of further self-assembled β-sheet structures that are highly resistant to hydrolysis.

. IR spectra of F-AX sample after the (a) 1 st , (b) 2 nd and (c) 3 rd run of pre-treatment in FA where the sample, in each run, was contacted with FA and heated at 160 °C for 2 h, then outgassed at bt. The spectrum of bare SiO2 obtained after outgassing at 160 °C and subsequent isotopic H/D exchange (by admission of 20 mbar D2O vapor followed by outgassing at bt) was subtracted as a baseline.

Discussion of Figure S2
IR spectroscopy measurements performed at the end of each run showed the formation of a significant band at around 1727-1731 cm -1 , a characteristic band of the presence of ester species on the surface, 3 which increased in intensity after each step.   Figure S4. Evolution of the amide I band intensity and Gly residues density (estimated based on TGA measurements) as function of time during Gly deposition by CVD over: G/AX(rt), G/AX and G/F-AX samples.

Discussion of Figure S4
Analysis of the integrated area (proportional to the concentration) of amide I versus time (h) allows to deduce that the Gly polymerization was much more efficient on G/F-AX sample compared to the other samples: higher values of the amide I integrated area were reached in shorter time on G/F-AX. The corresponding Gly residues density for the 3 samples at different CVD time is estimated based on the amount of peptide loading measured on the washed pellet by TGA that implies the amount of peptide loading to be 3.25% by weight after 20 h CVD. This corresponds to 6.9 Gly residues/ nm 2 . Figure S5. A) TGA and B) DTG profiles of the sample G/F-AX after washing with liquid water at the end of 20 h CVD reaction.

Discussion of Figure S5
The DTG profile recorded for the sample G/F-AX after washing with liquid water at the end of the 20 h CVD reaction shows a major event at around 280 °C. This corresponds to the burning off or destruction of the organic materials remaining on the surface. The weight loss between 130 and 400 °C represents 2.88 % by weight for this sample. When subtracting the weight loss of the corresponding blank silica (0.40 %), the actual peptide loading is estimated to be around 2.48 % by weight on G/F-AX after washing. This value corresponds to around 5.2 Gly residues/nm 2 . If washing with liquid water only allows solubilization of ca. 24% of the formed peptides (as stated in the main text, Figure 6) then before washing the peptide weight loading could be concluded to be 6.9 Gly residues/nm 2 after 20 h CVD. Figure S6. Second derivative in the amide I region of the double difference IR spectra of the intermediate CVD steps from 2.5 h (a) to 20 h (h) on G/F-AX, obtained by subtracting each step from the previous one.

Discussion for Figure S6
As discussed for Figure  The corresponding spectrum of the material obtained before the start of CVD process is subtracted as a baseline. Panel (II) shows the second derivative of the IR spectra a, and c.

Discussion of Figure S7
The general trends observed on G/AX ( Figure S7